Britain's outspoken foreign secretary has claimed that UK ally Saudi Arabia was engaging in "proxy wars" in the Middle East, comments that Prime Minister Theresa May said do not represent Britain's position.

British foreign minister Boris Johnson said on Thursday it was not "entirely right" to talk about a new Cold War with Russia because Moscow did not pose as much of a threat to global stability as the former Soviet Union had.

European Commission head Jean-Claude Juncker sharply criticised politicians Nigel Farage and Boris Johnson as the "sad heroes" of Brexit who resigned instead of leading Britain through the EU exit they campaigned for.

It would be hypocritical of President Barack Obama to support Britain staying in the European Union because Washington would never share its own sovereignty, London Mayor Boris Johnson said on Saturday ahead of a visit by the US leader.